The present invention relates to a device and to a method for laying tubular pipes underwater from a laying vessel.
Those skilled in the art make a distinction between flexible pipes and rigid pipes. Reference should be made to the American Petroleum Institute Specification API 17J for a definition of flexible pipes; in that flexible pipes have a relatively short (for example a few meters) minimum bend radius without plastic deformation, while rigid pipes, have a minimum bend radius without plastic deformation which is relatively large (for example several tens of meters). Furthermore, for the purposes of the description hereinbelow, the general term xe2x80x9cflexible pipexe2x80x9d will be used to cover not only true flexible tubular pipes, but also flexible risers, umbilicals and the flexible cables that a laying vessel may have to lay.
FR 2 660 402 A in the name of the assignee here of discloses a device and method for laying flexible pipe from the moon pool of a laying vessel. The pipe is paid out from a storage basket and sent over a large circular-arc-shaped guide gutter (affectionately known as a xe2x80x9ccamembertxe2x80x9d) arranged at the top of a medium-height vertical derrick surmounting the moon pool and supporting two multi-track tensioners which grip the pipe and move it along constantly. This system is well suited to flexible pipes, that is ones which can tolerate a fairly pronounced curvature both in the storage basket and over the return gutter and on the intermediate portions of the catenary-curve-shaped path. By contrast, this system is ill suited to rigid pipes which do not tolerate such curvatures or for which such curvatures give rise to permanent deformations.
In the case of rigid pipes, there are four main types of laying method.
According to the so-called S-lay technique, the lengths of pipe are joined together horizontally on the deck of the vessel and are made to follow a S-shaped path with a very large radius of curvature so as to prevent any plastic deformation. The pipe leaves the ship at a very oblique angle, over an inclined chute known as a stinger. This technique is suited only to small and medium laying depths because otherwise the weight of the catenary of pipe already laid could bend and cause the plastic deformation of the pipe at the point where it leaves the stinger. This is why other techniques have had to be developed for greater depths of water.
The so-called reeled pipe system is illustrated, for example, in GB 2 025 566 A, in which the pipe, which has been welded together as a great length (in excess of 1 km for example) on the dockside and reeled with plastic deformation around a reel located on the laying vessel, is paid out from a reel, again with plastic deformation, and passes through a straightener before being taken up by the multi track tensioner of an inclinable laying ramp located at the stern of the vessel and the inclination of which is chosen to suit the laying depth.
The so-called G-lay technique is illustrated, for example, in GB 2 287 518 A in which the lengths of pipe are joined together horizontally on the deck of the vessel as in the S-lay technique; the pipe formed is sent toward the bow of the vessel then returned astern by a large wheel giving rise to plastic deformation of the pipe, after which this pipe passes through straighteners and a succession of multi-track tensioners, and then is launched into the water at a certain inclination which may be chosen according to the depth of water so as to avoid plastic deformation of the pipe in the submerged catenary.
A variation on this method is disclosed in GB 2 296 956 A, one of the embodiments of which is akin to the so-called S-lay methods and envisages despatching the pipe, previously assembled in lengths on the deck, over an arched aligner placed at the top of an inclinable ramp equipped with tensioners or clamping jaws; the two inflections that the pipe experiences lead to plastic deformations of the pipe and make the use of straighteners necessary.
To avoid the plastic deformations imposed on the pipe in the G-lay or rigid reeled-pipe methods, the so-called J-lay methods have been developed, being characterized by the fact that the lengths of pipe are assembled on a vertical or slightly inclined tower of the laying vessel, the lengths thus being welded together in the direction in which the pipe is to be laid, thus avoiding any plastic deformation at this stage, the assembled pipe then being submerged to a great depth, still without plastic deformation. One example is illustrated in U.S. Pat. No. 5,464,307 which shows a device for laying tubular pipes from the moon pool of a laying vessel, the device comprising an inclinable tower arranged in and essentially a great height above the level of the moon pool, designed to place therein a certain length of pipe to be laid, and means for clamping the length of pipe and moving it along. However, according to that patent, the upper tower is a pipe-alignment tower consisting of a lightweight structure because it has to withstand only the weight of one section of pipe; it is continued under the level of the moon pool by a substantial support tower on which the said movement means are provided, these consisting of a moving carriage holding the pipe at one of its arrester flanges. Any interventions that might be required on the device are made complicated by the fact that it is, to a large extent, submerged.
In a variation of the above method, disclosed by McDermott Marine Construction at the Offshore Technology Conference held in Houston (US) in 1994, the moving carriage holding the pipe can move along the upper tower. However, as in the other version of the method, the pipe is held on the carriage by an arrester flange resting against a support block belonging to the carriage. Such flanges are not necessarily present on all types of pipe. Even when a pipe comprises flanges that limit the buckling, known as buckle arresters, welded at various points, these cannot be used to support the entire weight of the pipe without strengthening their welds, which are usually fairly lightweight, because they have then to withstand high shear forces requiring a weld with good penetration; this results in a high cost of preparation of the pipe.
The object of the invention is to provide a new pipe-laying device and its associated method, which do not exhibit the drawbacks of the prior art and which are particularly well suited to the laying of rigid pipes at very great depths (for example down to about 3000 m, and a maximum load of the order of 600 tons), while at the same time also allowing flexible pipes to be laid.
The device of the invention is, on the whole, of the family of J-lay devices. It comprises an inclinable tower arranged in and essentially a great height above the level of the moon pool, designed to accommodate therein a certain length of pipe to be laid, and means for holding the said length of pipe and moving it along; the said means comprise at least two moving carriages capable of travelling the said height of the tower and able to accommodate members for holding the pipe. These holding members are very advantageously members with clamping jaws, known as clamps. The intention here is to make a distinction between a true (and static) clamping device such as this and a tracked linear tensioner (entailing movement at the same time as clamping). The latter generally has an overall length which is far greater (for example 15 m in the case of the hauling forces involved here) than that of a clamping device of the clamp type (for example 4 m long). By controlling the clamping of the clamps and the movement of one or other carriages along the tower, it is possible to use the device of the invention in a great many possible configurations, giving the device great versatility. The support tower is constructed so that it can withstand the weight of the entire pipe already submerged, which can be suspended therefrom. Although the clamping jaws are the holding device of choice, it is not precluded for special applications to use the mobile carriages to hold bearing blocks collaborating with holding flanges on the pipe. In addition, it is also advantageous for the tower to be able to accommodate tracked tensioner devices, thus allowing it to be used in many configurations for laying flexible pipes, the carriages then generally being relegated to the top of the tower.
The carriages are individually motorized and driven, for example, by a rack and pinion system along the inclinable tower so that they can move while at the same time supporting the entire weight of the submerged catenary of pipe.
The welding between successive lengths is advantageously performed at deck level, as are the operations of non-destructive inspection and of coating the pipe in the region of the welds. During these operations, the upper length is held by the upper carriage or by auxiliary members provided on the tower or on the device used to get the said length into place, while the lower carriage holds the submerged catenary.
Advantageously, with a view to increase the possible applications of the device of the invention, the tower at its upper end comprises a flexible pipe return wheel mounted on a bracket immobilized on the tower. This then is no longer a vast chute intended to guide a moderately flexible pipe, but a return wheel for a truly flexible pipe.
Advantageously, the wheel bracket is mounted so that it can be moved along the tower and can be immobilized at different heights which, as will be seen later on, gives further diversity to the applications.
In a known way, the tower preferably comprises a winch and a service cable, or better still, a double winch and two cables, making many manipulations easier.
Advantageously, the height of the tower above the moon pool is made up of at least two disassemblable sections. In certain instances, the sections can be disassembled and just one of them used on another vessel.
The height of the tower above the deck of the vessel is, for example, of the order of 50 m or more, allowing lengths of pipe made up of four assembled segments (usually 48.8 m) to be constructed. Depending on the capabilities of the ship and on its stability, this number of four lengths may be altered.
According to an important feature of the invention, the tower comprises a runway over its entire length above the moon pool, and is associated with a device for conveying a length of pipe onto the tower, this device comprising an elongate chassis equipped with wheels capable of running, on the one hand, along the deck of the laying vessel and, on the other hand, along the said runway along which it is hauled by the winched cables or other auxiliary devices.
Advantageously, the tower is associated with a retractable work table that can be inclined with it, and which is capable of supporting the weight of the submerged pipe.
At its lower part, the tower comprises a roller box mounted on an articulated or retractable chassis.
The method of the invention uses the foregoing device in various configurations.
Thus, for laying rigid pipes, the two carriages are used hand over hand to successively grip and lower the lengths of pipe. The upper carriage is the only carriage to move, travelling the entire length of the tower, with the entire length of the submerged pipe catenary as it descends, while the lower carriage is static at the bottom of the tower and is used only to hold the catenary of pipe when it is not being clamped in the upper carriage.
Advantageously, the two carriages are raised back up to the top of the tower to install a connection end fitting at the tail end of the line, or to install other accessories such as manifolds or buoyancy modules. The operations of fitting the connection end fitting and of holding the submerged pipe are made easier by the presence of a retractable work table over the moon pool around the pipe, this work table being designed to support the weight of the said submerged pipe which is temporarily attached thereto.
According to the invention, the wheel may be used at the full height of the tower or at mid height for laying a flexible pipe, if necessary using tensioners fitted to the lower part of the tower.
It is also possible to use just one part of the tower on a conventional vessel for laying a flexible pipe.